Pile driving utilizing standing wave vibrations

ABSTRACT

A pile is driven into the earth by means of vibrational energy which is applied to the pile near one end thereof, at a frequency such as to set up resonant standing wave vibration in the pile. Means are provided to assure that the standing wave pattern has a velocity node shifted toward the driving end thereof and a velocity antinode near the driven end thereof. This end result is achieved by first driving the pile into the ground a short distance with a limited amount of vibrational force and with limited bias force being applied by the pile against the earth. When the pile has penetrated to a depth where lateral vibrations thereof are damped by the earthen material, the vibrational drive power is increased to cause the standing wave vibration with velocity nodes and antinodes as mentioned above.

22 Filed:

United States Patent [191 Bodi ne [54] PILE DRIVING UTILIZING STANDINGWAVE VIBRATIONS [76] Inventor: Albert G. Bodine, 7877 Woodley Ave., VanNuys, Calif. 91406 FORElGN PATENTS OR APPLICATIONS 387,473 2/1933 GreatBritain 173 49 [451 May 7,1974

Primary Examiner-Jacob Shapiro Attorney, Agent, or FirmEdwardA.'Sokolski 57] ABS'IRACT A pile is driven into the earth by means ofvibrational energy which is applied to the pile near one end thereof, ata frequency such as to set up resonant standing wave vibration in thepile. Means are provided to assure that the standing wave pattern has avelocity node shifted toward the driving end thereof and a velocityantinode near the driven end thereof. This end result is achieved byfirst driving the pile into the ground a short distance with a limitedamount of vibrational force and with limited bias force being applied bythe pile against the earth. When the pile has penetrated to a depthwhere lateral vibrations thereof are damped by the earthen material, thevibrational drive power is increased to cause the standing wavevibration with velocity nodes and antinodes as mentioned above.

3 Claims, 2 Drawing Figures PILE DRIVING UTILIZING STANDING WAVEVIBRATIONS I This invention relates to the use of vibrational energy indriving piles, and more particularly to a pile driving method andapparatus in which an optimum standing wave vibration pattern isdeveloped.

In my U.S. Pat. No. 2,975,846, an acoustical technique and apparatus fordriving piles is described in which a pile is vibrationally elasticallydriven by means of an orbiting mass oscillator at a frequency which setsup a resonant standing wave vibration pattern in the pile. In thisdevice, the vibrational energy is applied to the pile such that avelocity antinode of vibration, i.e., a region of maximum velocity andamplitude of vibration, appears near the upper end of the pile, thedriving end of the pile generally being biased strongly against theearth so that there is very often a velocity node at or near thisposition. Thus, in the device of my US. Pat.

No. 2,975,846, the top end of the pile elastically ,vi-

brates with a relatively large stroke, while the bottom end thereoftypically has a relatively small vibrational stroke.

It has been found that there are several disadvantages to operating apile driver with this type of vibrational pattern. First, the highvelocity vibration in the higher portions of the pile fully exposed tothe air tend to generate a good deal of noise which is obviouslyundesirable. Further, the high amplitude vibration at the upper end ofthe pile tends to cause considerable vibration of the surroundingstructures from which the pile is subtended, as well as in theoscillator drive and isolation mechanism, resulting in wear and tearthereon and the undesirable dissipation of vibrational energy. Further,where it is sometimes desired to use high impedance oscillators, such asthose of the piezoceramic type, for driving the pile it is easier. tomatch this type of vibrational source to a high impedance point in thestanding wave vibrational pattern rather than to the high velocity, lowimpedance afforded by a velocity anitnode. It is thus desirable to havea velocity node near the top end of the pile where the oscillator can beconveniently coupled thereto.

The technique and apparatus of this invention overcomes theaforementioned shortcomings of the prior art by making the wave patternin the pile to be exactly opposite to that of my aforementioned patent,that is, by making the velocity node (or pseudo node) appear above thecenter of the pile with the top end of the pile moving elastically withonly a small stroke, and the bottom end of the pile moving elasticallywith a large stroke. It is to be noted that in this type of system, thepile itself does not move up and down bodily but rather it shortens andlengthens elastically with the bottom end shortening and lengtheningappreciably more than the top portion does. In this manner, the higherup portions of the pile which would tend to radiate sound most readily,have relatively low amplitude vibration and thus the noise issignificantly lessened. Also, this has the additional advantages oflessening wear and tear on the parts directly coupled to the pile inview of the lower velocity vibration, and facilitating the coupling ofthe pile to a high impedance vibrational energy source.

It is therefore an object of this invention to reduce the noise involvedin vibrationally driving piles into the earth.

It is another object of this invention to facilitate the installation ofpile members in earthen formations.

It is still a further object of this invention to lessen the wear andtear and dissipation of energy in members utilized in conjunction withsonic pile drivers.

It is still a further object of this invention to facilitate thecoupling of a high impedance vibrational driver to a pile member to bevibrationally driven into the earth.

Other objects of this invention will become apparent from the followingdescription taken in connection with the accompanying drawings, ofwhich:

FIG. 1 is an elevational view illustrating the practice of the techniqueof the invention, and

FIG. 2 is an elevational view partially in cross section illustrating anembodiment of the invention utilizing an electroacoustic sonic energysource.

It has been found mosthelpful in analyzing the operation of the deviceof the invention to analogize the acoustically vibrating circuitinvolved to an equivalent electrical circuit. This sort of approach toanalysis is well known to those skilled in the art and is described, forexample, in Chapter 2 of Sonics by Hueter arid Bolt, published in 1955by John Wiley and Sons. In making such an analogy, force, F, is equatedwith electrical voltage, E; velocity of vibration, u, is equated withelectricalcurrent, i; mechanical compliance, C,,,, is equated withelectrical capacitance, C,.; mass, M, is equated with electricalinductance, L; mechanical resistance (friction), R,,,, is equated withelectrical resistance, R; and mechanical impedance, Z,,,, is equatedwith electrical impedance, Z,..

Thus, it can be shown that if a member is elastically vibrated by meansof an acoustical sinusoidal force, F sinwt (to being equal to 271' timesthe frequency of vibration), that Where mm is equal to (l/wC a resonantcondition exists, and the effective mechanical impedance, Z,,,, is equalto the mechanical resistance, R the reactive impedance components wM andl lwC cancelling each other out. Under such a resonant condition,velocity of vibration, u, is at a maximum, power factor is unity, andenergy is most effici'ently delivered to a load to which the resonantsystem may be coupled.

Just as in electrical circuitry, maximum acoustical energy can betransferred where a good impedance match exists, i.e., where the twoelements between which the energy transfer occurs have like impedances.This fact becomes significant in the device and method of this inventionin the transfer of energy from the driving end of a mandrel, casing orpiling to the earthen formation in effecting the driving operations.Thus, in view of the resistive impedance characteristics of earthenformations it is desirable that the driving end of the piling exhibitcorresponding good impedance characteristics with large stroke to assuremaximum transfer of energy in fluidizing the formation.

It is to be noted that in the device of this invention the mass andcompliance for forming the elastically vibrating system are furnished bythe structural members of such system themselves such that the earthenformation is not incorporated in such system. The earth under suchconditions acts as a friction load which provides no significantreactive components. This results in a random vibration of the earthenparticles near to the pile, rather than a lumped coherent vibration,with a considerable relative motion between the separate grains. The netresults is a high degree of fluidization' of the earthen structure witha thixotropic effect facilitating the penetration of the driven memberand the eventual compaction of such structure around such member as thesonic energization is smoothly terminated.

It is also important to note the significance of the attainment of highacoustical Q" in the casing 'member being driven, to increase theefficiency of the vibration thereof and to provide a maximum amount ofenergy for the driving operation. As for an equivalent electricalcircuit, the of an acoustically vibrating circuit is defined as theratio of the energy stored in each elastic vibration cycle to the energyused in each such cycle. Q is mathematically equated to the ratiobetween wM and wR,,,. Thus, the effective Q of the vibrating circuit canbe maximized to make for highly efficient, high amplitude vibration byminimizing the effect of friction in the circuit and/or maximizing theeffect of mass in such circuit.

Briefly described, the invention is as follows: A sonic vibrationgenerator is coupled to a pile to be driven into the earth near one endthereof. The opposite end of the pile is placed against the earth with alimited amount of bias force between it and the earthen material. Theoscillator is then energized to drive the pile with a limited amount ofvibrational force to cause the end of the pile to penetrate into theearth a short distance below the surface to a position where lateralvibrations of the pile are effectively damped by the fluidized earthenmaterial surrounding the pile. The oscillator is then driven to providehigh vibrational energy to the pile and to cause longitudinal resonantstanding wave vibration thereof with a velocity node of the standingwave vibration pattern above the center of the pile and a velocityantinode near the driving end of the pile. This end result can beachieved in several manners, including the adjustment of the vibrationfrequency and/or the coupling of a relatively large mass to the top ofthe pile to provide a high impedance load thereat. The pile is thusdriven to the desired depth with high velocity and displacementvibration in the standing wave pattern appearing primarily in theportions of the pile in the earth or close to the surface thereof sothat noise and the transfer of high amplitude vibrational energy tosurrounding parts of the pile rig are minimized.

Referring now to FIG. 1, a first embodiment of the device of theinvention is illustrated as being utilized in performing the operationof the method of the invention.

The device of the invention is utilized to drive a pile in earthenformation 11 in the following manner: With pile being tightly held bymeans of clamp12, the entire assembly is lifted into position fordriving by suitable means, such as a crane assembly A which operatescables 18. With pile l0 vertically positioned with its end portionlightly abutting against the earthen formation to be penetrated,oscillator 12 is first turned on so as to .cause low power rotation ofits eccentrically weighted rotor members as described inconnection withmy US. Pat. No. 2,976,846. It is to be noted that in the event that ahollow pile is used, the end portion thereof should sometimes be cappedto prevent the entry of unconsolidated earthen material therein.

The rotors are position phased as indicated in the aforesaid US. Pat.No. 2,975,846, and when rotatedin opposite directions producevibrational forces that are additive longitudinally and whicheffectively cancel each other out insofar as transverse vibration isconcerned. Pile 10 is thus longitudinally excited in accordance withthis invention at a frequency determined by the speed of rotation of therotors. When the end of the pile has penetrated into the earth a shortdistance, such that lateral vibrations thereof are damped out by thefluidized earthen material, this rotation speed is adjusted so as toproduce increased vibration of pile 10 with a standing wave as indicatedby graph lines 68 being set up therealong. As indicated by graph lines68, maximum resonant vibrational activity is provided at the lowerportion of pile 10 where it is most needed to accomplish the drivingoperation. Frequency adjustment for this new wave pattern can be furtheraided by having substantial mass in element 12.

Pile 10 is preferably fabricated of a material such as steel havingrelatively high Q characteristics to make for a resonant system havingoptimum efficiency.

During this operation, the earthen particles are fluidized and compactedso that they in effect stand back from the pile permitting relativelyeasy penetration. In view of the fact that the elastic vibration systemis contained fully within the pile and the associated components havethe wave pattern shown, the earthen material being penetrated tends toact as a resistive load iwth the particles being caused to vibraterandomly rather than as a unitary mass. It is believed that each of theindividual irregular grains when energized by the sonic energy in thisfashion separately vibrates in a random path with a relatively fixedradius of vibration which changes in direction but remains fixed inmagnitude. Such random vibration effectively separates the irregularparticles so that they do not adhere to each other and are thuskept in ahighly fluid or thixotropic condition. This high degree of fluidity,with the large amplitude at the bottom of the pile as indicated by 68,not only greatly eases the penetration of the pile member but alsoconditions the soil surrounding the pile so that highly effectivelodgment of the casing or pile member driven therein is possible.

The pile can be effectively driven if it is excited by elastic vibrationwithin the pile itself, again with the characteristic wave patternhaving more amplitude at the penetrating end than it has at the top.This special kind of pattern has the node (or pseudo node) elevated uphigh, above the center, so that the lower active portion is more thanone half the total pile length. In this way the major important portion,the lower portion, is continually active. A very desirable fastpenetration can be accomplished in this way.

Another advantage from my invention is that this particular wave patternhas a large acoustic impedance at the top, which is greater than theacoustic impedance at the bottom end. Fortunately the large acousticimpedance at the top end can be accomplished by this standing wavepattern, without making it necessary to taper the pile to get largeacoustic impedance. The pile thus can be a straight uniform column.

By having this large acoustic impedance at thetop end I am then able toprovide good impedance matching with a high impedance electroacoustictransducer,

which is useful in certain applications. Examples of such high impedancetransducers are magnetostriction metals which vibrate from a pulsatingelectromagnetic field, or better even than this, there areelectroacoustic transducers that are made from electrostrictivematerials such as lead-zirconate-titanate which vibrate in response to apulsating electric voltage.

Referring to FIG. 2, we see the elastic pile 70 with electroacoustictransducer 72 firmly connected to the top of the pile by means ofhydraulic clamp 74. A heavy mass 76 on the other side of theelectroacoustic transducer 72, as well as clamp structure 74, helps toprovide the desired wave pattern by providing a high impedance at thetop of the pile 70. Electric power source 75 provides the pulsatingelectric power to drive the electroacoustic transducer 72.

The feature of this invention which provides the very surprising andbeneficial result comes from the frequency of the electricity from thepower source 75. This frequency is chosen to generate a standing elasticwave pattern in the pile, with the nodal region N above the center, asshown by wave pattern graph 78.

As will be noted from the figure, at the operating frequency of myinvention the pattern has more amplitude at the bottom than at the top.Therefore it has greater impedance at the top. This matches up nicelywith the electroacoustic transducer 72, as well as having the otherbenefits of high performance and low noise mentioned hereinabove. I I

I claim:

1. A method of driving a pile member into the earth comprising the stepsof:

placing one end of the pile against the surface of the earth with alimited amount of bias force thereagainst so that the earth will presentonly a moderate impedance load when the pile is subsequently driven,

driving said pile with a limited amount of vibrational force by meansofa sonic oscillator until said one end of the pile has penetrated intothe earth a short distance at which lateral vibrations of the pile aredamped by the earth,

operating said oscillator at a frequency such as to cause resonantstanding wave vibration of the pile with a velocity antinode of saidstanding wave appearing near said one end of the pile and a velocitynode thereof above the center so as to be more near the other endtherof, and

increasing the power to said oscillator to effect the penetration of thepile into the earth to a desired depth.

2. The method of claim 1 and further including the step of attaching amember having substantial mass to the other end of said pile tofacilitate the formation of a velocity node near said other end.

3. A system for use in driving a pile or the like into the earthcomprising:

a column member having elastic compliance in its longitudinal dimension,

guide and support means connected to said column .member for orientingsaid column member for longitudinal penetration of one end thereofprogressively into the earth, said guide and support means controllingthe bias force of the column member against the earth,

a substantial mass connected to said column at the other end thereof,

a vibration generator connected to said column, and

means for driving said vibration generator at a longitudinal resonantfrequency of said column so as to set up a standing vibration wavepattern in said column at said frequency, said standing wave having anodal region closer to said other end of the column than the length ofaquarter wavelength of the standing wave pattern.

1. A method of driving a pile member into the earth comprising the stepsof: placing one end of the pile against the surface of the earth with alimited amount of bias force thereagainst so that the earth will presentonly a moderate impedance load when the pile is subsequently driven,driving said pile with a limited amount of vibrational force by means ofa sonic oscillator until said one end of the pile has penetrated intothe earth a short distance at which lateral vibrations of the pile aredamped by the earth, operating said oscillator at a frequency such as tocause resonant standing wave vibration of the pile with a velocityantinode of said standing wave appearing near said one end of the pileand a velocity node thereof above the center so as to be more near theother end therof, and increasing the power to said oscillator to effectthe penetration of the pile into the earth to a desired depth.
 2. Themethod of claim 1 and further including the step of attaching a memberhaving substantial mass to the other end of said pile to facilitate theformation of a velocity node near said other end.
 3. A system for use indriving a pile or the like into the earth comprising: a column memberhaving elastic compliance in its longitudinal dimension, guide andsupport means connected to said column member for orienting said columnmember for longitudinal penetration of one end thereof progressivelyinto the earth, said guide and support means controlling the bias forceof the column member against the earth, a substantial mass connected tosaid column at the other end thereof, a vibration generator connected tosaid column, and means for driving said vibration generator at alongitudinal resonant frequency of said column so as to set up astanding vibration wave pattern in said column at said frequency, saidstanding wave having a nodal region closer to said other end of thecolumn than the length of a quarter wavelength of the standing wavepattern.